Unity input displacement factor frequency changer

ABSTRACT

The invention relates to a frequency changer (or cycloconverter) system employing conduction controlled bilateral switches operating in a controlled switching mode to maintain a unity input displacement factor for the frequency changer system independent of load power factor and output voltage. The input displacement factor, which is often referred to as the input power factor, of the frequency changer system is defined to be the angle between the input, or source, voltage and the fundamental component of the input current drawn by the frequency changer system.

United States Patent Gyugyi 1 51 Dec. 26, 1972 [54] UNITY INPUT DISPLACEMENT FACTOR FREQUENCY CHANGER [72] Inventor: Laszlo Gyugyi, Penn Hills, Pa.

[73] Assignee: Westinghouse Electric Corporation,

8 Pittsburgh, Pa.

[22] Filed: Nov. 12, 1971 [21] Appl. No.: 198,215

Related US. Application Data [63] Continuation-in-part of Ser. No. 84,796, Oct. 28,

[52] US. Cl. ..321/5, 321/7, 321/69 R [51] Int. Cl. ..H02m 5/22 [58] Field of Search ..321/5, 7, 69 R [56] References Cited UNITED STATES PATENTS 3,l70,107 Jessee ..321/7 X 3,178,630 4/1965 Jessee ..321/7 3,419,785 12/1968 Lafuze 21/5 x 3,431,483 3/1969 Lafuze.... ..321/7 3,470,447 9/1969 Gyugyi 6161. ..321/7 3,493,838 2/1970 Gyugyi el al. ..321/7 Primary Examiner-William M. Shoop, Jr. Attorney-F. H. Henson et al.

" 57 ABSTRACT The invention relates to a frequency changer (or cycloconverter) system employing conduction controlled bilateral switches operating in a controlled switching mode to maintain a unity input displacement factor for the frequency changer system independent of load power factor and output voltage. The input displacement factor, which is often referred to as the input power factor, of the frequency changer system is defined to be the angle between the input, or source, voltage and the fundamental component of the input current drawn by the frequency changer system.

3 Claims, 19 Drawing Figures BILATERAL 4e SWITCH co-0ucno- CONTROL CIRCUIT MTENTEUnacze m SHEET UlflF 16 Q0! BILATERAL 3 SWtTCH CONDUCTION CONTROL CIRCUIT FIG. I

L 19 LOAD E L c VIA V 003 VIC 18 PATENTED 05626 I97? 3. 707,666

SHEET 02 or 16 OONDUCTION i CONTROL CIRCUITII III CLOCK STAGE 2 CHANNEL 11 2 'GE N RiEOR RING AMPLIFIEE Pl J FouNTEF H3 3 3 l STAGE2 CHANNEL i? RING AMPLIFIER COUNTER l3 CONDUCTION CONTROL ClRCUlT I CONVERTER I Q03 1 VA LOAD L 2 CONVERTER I[ PATENTED B 1912 3. 707.666

SHEET 0 6 HF 16 BILATERAL SWITCH CONDUCTlNG CONTROL CIRCUIT FIG. 5A.

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SHEET 12 [1F 16 |0O\ 1 -v 3-PULSE I6 sINE wAvE s CROSSING 3 C CONTROI. STAGE 3 (NEGATIVE SLOPE) RING CHANNEL ,I2

COUNTER AMPLIFIER *-I4 |O2\ V H 3-PULSE %'I=I IS1RI% CONTROL I,

(NEGATIVE SLOPE) P --4I| STAGE 3 RING CHANNEL -I COUNTER AMPLIFIER -II- -0I5 |04 IIz I20 -v 3-PULSE Q sINE wAvE 5 CROSSING III C- CONTROL 1 (POsITIvE SLOPE) 3 I06 l hfG 3 H5 I -CIIANNEI COUNTER AIII IATE v 5H CROSSING 112 c H CONTROL 113 (POSITIVE SLOPE) -oII6 STAGE 3 RING CHA-NEL 1I4 I08 COUNTER AMPLIFIER sINusOOIAL 4 REFE NCE WA GENERATOR pATimEnnzczsmz 3.707.666 I SHEET 13UF 16 VB VC VA VR u) I OH I (2) I I I I I I I I (3)16 I I I I I I (4)12 I I I I I I I (5) 14 I I I I I I VA VB VC VR (7) 12 I I I I I I I I I I I (8) II I L I I (9)13 I I I I I I I (|O)15 I I I I I VBA VCA ca VAB VAC ac VOA! UNITY INPUT DISPLACEMENT FACTOR FREQUENCY CHANGER CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation in part of application Ser. No. 84,796, entitled Unity Input Displacement Factor Frequency Changer, filed Oct. 28, 1970. This application is also related to a second cofiled continuation in part application Ser. No. 198,283 of application Ser. No. 84,796.

BACKGROUND OF THE INVENTION In conventional direct AC to AC frequency changer systems the input displacement factor of such systems is dependent upon the power factor of the load at the output. The relationship between the input displacement factor and the loadpower factor may be linear or non-linear depending upon the operating principle of a particular frequency changer. For example, the naturally commutated (or phase controlled) frequency changer always has a lagging input displacement factor independent of the load power factor. The input displacement factor of the naturally commutated frequency changer is thus non-linearly related to the load power factor such that a unity power factor load, or resistive load, is seen by the power source as an inductive load, and a purely capacitive load or a purely inductive load as seen by the power source as a pure inductance. In the case of the force commutated mode of operation of a frequency changer, as described in US. Pat. No. 3,493,833 entitled Static Frequency Converter with Novel Voltage Control, which is assigned to the assignee of the present invention, there is a direct linear relationship between the load power factor and the input displacement factor, i.e., the input displacement factor is equal to the load power factor.

In many applications it would be advantageous to have a frequency changer system with a load-independent, unity input displacement factor such that the input power source would be required to supply only the resistive component of the load current, i.e., the current portion which is actually consumed. ln applications where the input power is produced by a local generator this would result in a weight and size reduction for the generator and electrical cables, as well as providing improved overall efficiency. In other applications such as high power AC motor drives, a frequency changer system having a unity input displacement factor would reduce installation and operation expenses.

SUMMARY OF THE INVENTION The novel frequency changer systems comprising this invention is a unity input displacement frequency changer system which draws only the resistive component of the load current from the power source regardless of the power factor of the load. The unity input displacement factor characteristic of the frequency changer system is maintained for any load ranging from a pure capacitance to a pure inductance. In other words the rated inductive or capacitive load current can be supplied without actually drawing any fundamental current from the input power source.

The unity input displacement factor mode of opera tion of a pair of converters comprising a frequency changer system is described and illustrated with reference to two independent configurations. In a first configuration two converters employing bilateral switches being supplied from the same input power source are operated such that the one of the converters transfers back tothe input terminals the actual load power factor while the other converter transfers back an inverted representation or mirror image of the actual load power factor. The combination of the actual and inverted load power factor at the input terminal results in a zero reactive load current flow through the input power source and thus establishes a unity input displacement factor for the frequency changer system.

In the second configuration each of the converters exhibits a unity displacement factor and the combination of the two converters results in a substantial reduction in input current and output voltage distortion while maintaining a unity input displacement factor. The output voltage waveforms of each of the converters are generated by appropriately phase modulating the firing angles of the bi-lateral switches in each converter with respect to the phases of the supply voltages. The modulation is controlled so as to produce complementing output wave shapes from the converters with each exhibiting the same mean output voltage. This is achieved by maintaining the sum of the firing angles of the two converters equal to It is noted that this method of basic control is essentially the same as that used for controlling the firing angles of a naturally commutated frequency changer. Thus, existing control techniques, such as the sine wave crossing method, or integral lining control method can be used to obtain the proper phase modulation. The similarity ends at this point, however, in that the operation of the frequency changer system comprising this invention is completely different from that of the naturally commutated frequency changer. The naturally commutated frequency changer consists of a positive conducting converter and a negative conducting converter such that each converter normally operates only during one-half cycle of the output current and remains idle during the other half cycle. The two converters of the naturally commutated frequency changer can be maintained in continuous conduction only if a circulating current is allowed to flow between them via the input supply source. In either of these modes of operation, however, the input displacement factor of the naturally commutated frequency changer is less than unity.

The frequency changer system comprising the second configuration consists of two converters employing bilateral switches. Each of the converters is capable of conducting both positive andnegative current. Thus both converters are in conduction during the total output cycle, each one supplying half of the load current. There is .no significant circulating current between the converters and both converters exhibit unity displacement factors at their input.

In both configurations where two converters are employed the total rms distortion of the input current as well as the rms distortion of the output voltage is significantly reduced.

The invention will become readily apparent from the following exemplary description in connection with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a basic schematic illustration of a frequency changer utilizing bi-lateral switching elements;

FIGS. 2A and 2B are waveform illustrations of two switching modes of operations for the bi-lateral switching elements of FIG. 1;

FIG. 3A is a schematic illustration of a frequency converter system utilizing two frequency converters connected in parallel by means of interphase reactor;

FIG. 3B is a waveform illustration of the operation of the embodiment of FIG. 3A;

FIG. 4A is a schematic illustration of an embodiment of the clock pulse generator of FIG. 3A;

FIG. 4B is a waveform illustration of the operation of the clock pulse generator of FIG. 4A;

FIG. 5A is a'schematic illustration of a six pulse frequency changer system comprised of interphase reactor coupled frequency changers as illustrated in FIG. 3;

FIG. 5B is a block diagram of a control circuit for a six pulse frequency changer system of the type illustrated in FIG. 5A;

FIGS. 6A and 6B are waveform illustrations of the operation of a frequency changer system of FIGS. 5A and 5B;

FIG. 7 is a graphical illustration of the operation of the six pulse frequency converter system of FIG. 5 according to the invention;

FIG. 8 is a schematic illustration of a three-phase output frequency changer system comprised of sixpulse bridge type frequency changers as illustrated in F FIGS. 9A is a graphical illustration of the operation of the frequency changer system of FIG. 8 according to an alternate embodiment of the invention.

FIG. 10A is schematic illustration of a control circuit of the type used in the embodiment of FIG. 8;

FIGS. 10B and 10C are waveform illustrations of the operation of the embodiments of FIGS. 8 and 10A; and

FIGS. 11A and 11B are waveform illustrations of an alternate mode of operation of the embodiment of FIG. 10A.

DESCRIPTION OF THE PREFERRED EMBODIMENT The novel configurations for achieving unity input displacement factor for frequency changer systems independent of load power factor and, input and output frequency, will be described separately. FIGS. 1-8 correspond primarily to the discussion of one configuration, while FIGS. 8-11, which are further extensions of FIGS. 1-8, correspond to the second configuration.

Referring to FIG. 1 there is illustrated schematically a frequency changer 10 comprised of generator G, bilateral switches QQ, and electrical load L. The bilateral switches QQ are typically represented as a diode bridge b in combination with a transistor t. The ON- OFF control of the transistors provided by control circuit 18 in combination with the current conducting paths provided by the diode bridges enables the bilateral switches 00 to conduct current in both the forward and reverse directions. The current conduction to load L from the generator phases A, B and C provided by the controlled conduction of the bilateral switches O01, Q02 and 003, respectively, develops the individual phase voltages across the load L.

According to the basic mode of operation of such a frequency changer as described in US. Pat. Nos. 3,148,323 issued to Blake et al. Sept. 8, 1964; 3,170,107 issued to Jessee Feb. 16, 1965; and 3,493,838 issued to Gyugyi et al. Feb. 3, 1970. These US. Patents, the latter two being assigned to the assignee of the present invention, are incorporated herein by reference. Each of the bilateral switches 00 are allowed to conduct for a fixed period of time T, such that the input voltages corresponding to the input generator phases A, B and C are successively connected to the load L for the same timer interval T, resulting in the fabrication of a predominantly sinusoidal output waveform across the load L. It is shown in the aboveidentified US. Pat. No. 3,148,323 that the fundamental component of the generated output voltage has a frequency f, =|f,,13 lf, where j}, equals HT, 3 represents the number of bilateral switches and corresponds to the pulse number of the converter and f, is the supply frequency. This expression indicates that there are two possible values of f,,, one greater and one less than 3f, which will result in the same output frequency. If f,,/3 is greater than f,, than the output frequency, f", is given by:

f0 =fa/ fl If f,,/3 is less than f,, then f", takes the following form L =fi -fp2/ (2) The output voltage waveshapes corresponding to the switching frequencies f and f respectively are shown in FIGS. 2A and 2B. It can be shown that the fundamental components of the two waveforms are identical and correspond to the following expressions:

A 010) o/ Q fo 1 (4) It can be shown that the phase relationship between an input phase voltage, i.e., V,, of the generator G and the dominant, or fundamental input current 1,, drawn by the frequency changer 10 corresponding to the switching frequencies f and f can be described mathematically as follows: if one of the input phase voltages is expressed as:

m 28mm then the current 1,,will be 1m= 3 fsbwiaf i iz n w 6) when the frequency changer is operated by the switching frequency f,, (i.e., f /3 f,), and the input current will be m fizwlof mfiw) (1) when the frequency change is operated by switching frequency f,,, (i.e., f,,,/3 f,). It is observed from the above equations (6) and (7) that while the magnitude of the input current remains unchanged for both switching frequencies (f,,, and f the phase angle of the input currents corresponding to the respective switching frequencies are opposite.

It can therefore be concluded that if a frequency changer operates at the frequency f, it will result in an input displacement angle of the I frequency changer which is the negative ofthe load phase angle, and if the frequency changer operates at the switching frequency f the resulting input displacement angle will be identical to the load phase angle. The amplitude of the input current at both switching frequencies remains the same. v

The significance of the above relationship of the current and voltage of the frequency changer operating at the switching frequencies f,, and f3, will become more apparent from the following discussion in reference to FIGS. 3A and 3B.

Referring to FIG. 3A there is illustrated a frequency changer system 20 comprised of supply voltage generator G, two converters 22 and 24, the outputs of which are connected in parallel via an interphase reactor 26 to the electrical load L. The ON-OFF conduction of the bilateral switches of converters 22 and 24 are controlled bia conduction control circuits 28 and 30 by clock pulse generator 32.

The operation of the frequency changer system 20 is illustrated by the waveforms (l)( 11) of FIG. 3B.

The clock pulse generator 32 provides twostrings of pulses P and P illustrated in waveforms (l) and (2) respectively, having frequencies of f,, and f respectively, as defined by equations (1) and (2). In order to ensure that the fundamental components of the two composite output waveforms V and V, illustrated in wave forms (9) and respectively are in phase, clock pulse waveP is shifted by one half of its period time, f with respect to clock pulse wave P as illustrated in FIG. 3B. The phase relationship between the two coordinated clock pulse waves and the supply voltages V V and V is somewhat arbitrary; the one shown in FIG. 3B was chosen for the purpose of clear illustration. The actual generation of clock pulse waves P, and P having the required frequency and phase relationship will be explained later.

Clock pulses P, and P are fed to-three-stage ring counters 34 and 36 which develops the ON-OFF conduction control waves. I1, I2, l3 illustrated in waveforms (3), (4) and (5) respectively and Ill, II2, II3 illustrated in waveforms (6), (7) and (8) respectively. These control waves are appropriately amplified by two three channel amplifiers, 38 and 40, and then coupled to the control electrodes of the bilateral switches of converters 22 and 24. In the waveform illustrations of FIG. 3B it is assumed that each bilateral switch becomes conductive whenever the corresponding control wave is positive, and it ceases conduction when the COI'ItIO] wave IS zero.

The above described sequential operation of hilateral switches QQl, Q02 and 003 of converter 22 age wave shape with the same fundamental components, it follows that the frequency changer system 20 will generate an output voltage wave shape V illustrated in waveform (ll) of FIG. 3B, which is the arithmetic means of the constituent waves V and V, as illustrated in waveforms (9) and (10) of FIG. 3B. In this mode of operation, therefore, the fundamental component of output waveform v is also defined by equation (3). 6

While the expression for the fundamental component of the voltage output waveform of the frequency changer system 20 remains the same as the expression for the fundamental component of the voltage output waveform of the frequency changer system of FIG. 1, the fundamental component of the input current for the frequency changer system 20 becomes mi+2 =(I1 1+ yz)/ where 01,, is the amplitude of the real component of the load current and is defined according to the equation: A

Zol -1 l cos 9 It is apparent from equation 8) that the operation of the converters 22 and 24 at the switching frequencies f,,,/3 and f /3 respectively isolates the input power source G from the reactive component of the load current thereby establishing a unity input displacement factor for the frequency changer system 20 regardless of load power factor and output frequency.

This novel operation of the combined frequency changers 22 and 24 of FIG. 3A provides an additional benefit in that the total rms distortion in the output voltage waveform is reduced as is apparent from a comparison between the component waveforms of FIG. 3B and the resultant waveform. It has been determined that this rms distortion is reduced by a factor of l/29\/2 As was explained above, in the described mode of operation of frequency changer 20 the two clock pulse waves P and P must have the previously defined frequency and phase relationship. It is obvious that there are numerous ways of implementing the operation of clock pulse generator 32 to produce the series of pulses required to operate the control circuits 28 and 30 in the desired manner. A possible implementation of the clock pulse generator 32, utilizing the well known principles of the sine wave crossing control, is shown in FIG. 4A and its operation is illustrated by the waveforms (l )(9) of FIG. 4B.

The clock pulse generator is comprised of transformers 322, 324, 326, producing three sinusoidal timing waves which are in antiphase with the supply voltages, a reference generator 328 generating a sinusoidal waveform of the wanted output frequency, three-pulse sine wave crossing detector circuits 330 and 332, an integrator 334, null detector 336, a flip-flop circuit 338, logic AND' gates 340, 342, 344 and 346 and logic OR gates 348 and 350.

The operation of the clock pulse circuit 32 may be explained as follows with reference to FIG. 4B.

The three timing waveforms, V,, V,, and V are compared to reference wave V In the present discussion it is assumed that the amplitude of the reference wave is approximately equal to, but not greater than the amplitudes of the timing waves. At the points of intersection of the reference wave with the negative 

1. An AC to AC frequency changer apparatus for developing an AC output voltage waveform across a load, comprising, in combination, an AC input power source, a converter circuit means including a plurality of ON-OFF conduction controlled bilateral switching means operatively connected between said AC input source and said load, each of said bilateral switching means being capable of conducting current in both directions between said input power source and said load, phase modulation conduction control circuit means operatively connected to said ON-OFF conduction controlled bilateral switching means to control the ciring angles of said bilateral switching means and sequentially render conductive said bilateral switching means in phase relationship with the alternating voltage supplied by said AC input power source, said bilateral switching means developing said AC output voltage waveform across said load, said bilateral switching means being rendered conductive by said phase modulation control conduction circuit means during both the positive and negative half cycles of the AC current flowing in said load such that the current drawn from said AC input power source is in phase with the corresponding phase voltage of said AC input power source thereby establishing a unity input displacement factor for said converter circuit means.
 2. Apparatus as claimed in claim 1 wheRein said converter circuit means includes a first converter circuit and a second converter circuit, said phase modulation control conduction circuit means providing phase modulated ON-OFF conduction control of the bilateral switching means of said first and second converter circuits with respect to the alternating voltage supply by said AC input power source to develop intermediate first and second output voltage waveforms respectively, said bilateral switching means of said first and second converter circuits being rendered conductive by said phase modulation control conduction circuit means during both the positive and negative half cycles of the AC output current flowing in said load to establish a unity input displacement factor for both said first and second converter circuits, said phase modulation control circuit maintaining the sum of the firing angles of said first and second converter circuit at approximately 180*, and means for combining the outputs of said first and second converter circuits, said combination resulting in reduced distortion of the current drawn from the AC input power source and reduced distortion of the AC output voltage waveform while maintaining the unity input displacement factor for said converter circuit means.
 3. An AC to AC frequency changer apparatus as claimed in claim 1 wherein the average variation of the phase angle over a full output cycle established by said phase modulation conduction controlled circuit means is approximately zero. 